The Animal Barrier Shared Resource Facility (ABF) provides for pathogen-free mice while permitting supervised access for investigators, stable husbandry conditions, and individualized daily health care for animals. This is complemented by comprehensive rodent quality assurance and quarantine programs that allow acquisition and maintenance of both pathogen-free and infected rodents. Four specialists in laboratory animal medicine provide veterinary care and a Board-certified veterinary pathologist provides diagnostic services. These veterinarians also provide oversight for animal husbandry and regulatory requirements, and intensive small-group training of facility users. The number of AECCC investigators who use the barrier facility has increased from 13 in early 1996 to 33 as of May 2000 and the daily census of cages in the Chanin Building has increased from 2500 to over 6600 between October 1994 and May 2000. All mouse caging used in the ABF has been upgraded, replaced, and increased over the past four years. The cage wash area was renovated and new equipment installed. The frequency and quality of sentinel mouse exposure and testing has been improved and investigators are now routinely informed of test results. When infectious agents are detected in an animal barrier room, prompt and definitive action is taken to isolate the suspect animals, confirm the infection, and treat or relocate infected animals with minimal interference to ongoing research. Multiple, incremental improvements have been made in operating procedures to optimize quarantine for animals arriving from other institutions allowing investigators to import animals with a desired genotype, re-derive the strain by embryo transfer or other methods, and verify pathogen elimination with minimal delays. Methods have been implemented which allow maximal retention of animals when an infectious agent is detected in a barrier room. These include static microisolator housing, handling animals with disinfected forceps, opening animal cages only in a hood and requiring that full barrier garb (gown, cap, mask, booties, gloves) be worn by all persons entering animal rooms. Cage-by-cage testing methods have been implemented which remove only exposed animals from the colony. When infected colonies include immunodeficient or partially immunodeficient animals such as scid, rag, or op/op mice, each cage of immunodeficient mice is tested for the presence of the pathogen by co-housing immunodeficient animals with immunocompetent sentinels.